Fluid-enhanced coarsening of mineral microstructures in hydrothermally synthesized bornite-digenite solid solution

Abstract

Symplectic microstructures are abundant in copper−iron−sulfide minerals and are conventionally considered to form by solid-state diffusion processes. Here we experimentally demonstrate that coarsening of exsolution lamellae occurs ∼1000 times faster in the presence of a fluid compared to the equivalent dry system. Bornite-digenite solid solutions (Cu₅FeS₄−Cu₈.₅₂Fe₀.₁₁S₄.₈₈) were synthesized hydrothermally via the replacement of chalcopyrite, and we compared the microtextures in the product subjected to different cooling histories: (i) dry annealing after synthesis; (ii) cooling to an annealing temperature immediately following hydrothermal synthesis; and (iii) annealing in a hydrothermal fluid following quenching to room temperature and then reheating. We interpret the rapid coarsening of the exsolution lamellae in the presence of a fluid phase to result from recrystallization associated with healing of the open porous microstructure in the parent phase. The porosity is a consequence of the synthesis of the parent bornite−digenite solid solutions via interface coupled dissolution reprecipitation. The texture coarsening is accompanied by the destruction of the transient open porous microstructure via coalescence of the pores and their migration to lamellae and grain boundaries. As a result, the final microstructure and the kinetics of textural coarsening depend upon the crystallization and cooling history of the parent mineral. Such fluid-driven textural evolution may be a major mode of reaction in ore systems, and is likely to affect oxide and silicate systems alike in the presence of aqueous fluids.